Thick dolostone units form a cuesta that dips gently toward Lake Michigan, forming the
Door Peninsula. South of Lake Winnebago the edge of the Silurian rocks is mantled by thick
glacial deposits. Silurian rocks also occur on the summit of Blue Mound west of Madison.

The Niagara Escarpment, the edge of the Silurian rocks, wraps around the Michigan
Basin, It defines the east shore of Lake Winnebago and the Door Peninsula in Wisconsin and
the Garden Peninsula in upper Michigan. It is mantled by thick glacial deposits north of
there but emerges on Manitoulin Island in Lake Huron. It forms the Bruce Peninsula that
separates Lake Huron from Georgian Bay. The Bruce Peninsula is a near twin of the Door
Peninsula, although much less developed. The Escarpment then runs across Ontario and east
into New York. Niagara Falls flows across it, giving it its name.

South of Lake Winnebago, the Silurian escarpment is largely concealed by glacial
deposits. It is also much less streamlined by ice scour and has a much more ragged outline
on the map. It forms a distinct landform in places in Illinois and Iowa.

The Niagara escarpment is purely erosional, marking the edge of a thick, resistant
series of rock layers. It is not connected to any fault or other tectonic disturbance.